This invention relates to a method for the fuel supply and a fuel supply system for aircraft equipped with at least one aero gas turbine.
The fuel supply systems of aircraft are provided with suction delivery systems besides the aircraft-side tank boost pumps. These systems remain operative even in the event of a total failure of the aircraft electrical power supply, i.e. in a situation in which the tank boost pumps are no longer available. In the fuel supply systems according to the state-of-the-art, jet pumps are used with aero gas-turbine propulsion systems in order to increase the suction delivery capacity, these pumps removing the out-gassed air present in the fuel system from the aircraft-side fuel supply system. Out-gassing of the air is due to the pressure decrease in suction operation, among others.
The ejector jet of such a suction pump is normally supplied by the engine-side low-pressure fuel pump. Here, the out-gassed air in the fuel line is sucked by the jet pump, mixed with the ejector jet to form small bubbles and then fed into the inlet of the low-pressure fuel pump. These small bubblesxe2x80x94evenly distributed in the fuelxe2x80x94enhance the suction delivery capacity of the pump impeller. It should be noted, that complete separation of air and fuel into two phases is shifted towards higher flight altitudes.
A disadvantage of the previous systems lies in the fact that the performance of these jet pump systems, which are used to increase the suction delivery capacity in turbine propulsion systems, is limited, this circumstance being due to operational dependence of these systems on the delivery capacity of the low-pressure fuel pump. As the delivery pressure of the low-pressure fuel pump is varying in an irregular way in suction operation at higher flight altitudes, the suction capacity of the jet pump inevitably decreases until a point is reached at which no fuel is sucked at all.
A further disadvantage is the heating of the fuel in the low-pressure fuel pump. This heating results from the losses occurring in the fuel circulation of the jet pump ejector jet supply around the pump. The resultant heat inevitably increases the vapor pressure of the fuel. Accordingly, the boiling range of the fuel is reached at relatively low flight altitudes.
In order to avoid the above-mentioned problems, the diameters of the state-of-the-art fuel supply lines to the engine are selected as large as possible, thereby providing for minimal line pressure losses. Consequently, with the pressure being decreased, out-gassing of the air dissolved in the fuel can be minimized in suction operation. A major disadvantage of this approach lies in the fact that the large line diameters involve low flow rates. As a consequence, the out-gassed air accumulates at the line high-points, for example in the main landing gear bay, and is not carried away with the fuel. Consequently, this air cannot be managed by the engine-side jet pump and, in extreme cases, will fill the aircraft-side fuel supply line over its entire length. This is particularly critical for stern-powered aircraft, where the entire air quantity may abruptly move to the engine and cause the delivery flow of the engine-side low-pressure fuel pump to collapse, for example when the aircraft nose is pulled down for descent. This will result in flame-out of the combustion chamber and, consequently, in blow-out and run-down of the engine.
A broad aspect of the present invention is to provide a method and a system for fuel supply which ensures the safe fuel supply of aero gas turbines while avoiding the disadvantages of the state-of-the-art.
It is a particular object of the present invention to provide remedy to the above problems by the features cited in the independent claims, with further objects and advantages becoming apparent from the sub-claims.
Accordingly, the present invention provides for an increased flow rate in the fuel line to the engine. This results in considerable advantages. Although the increased flow rate inevitably involves higher flow losses, the inlet conditions at the engine actually are improved. In contrast to the situation known in the state-of-the-art, in which the engine-side low-pressure fuel pump would practically be overwhelmed by a sudden, large quantity of air, the increase of the flow rate as provided by the present invention enables the air to be delivered continually and in manageable quantities to the low-pressure fuel pump and, subsequently, to be further compressed in the high-pressure system. Thus, undesired accumulations of large quantities of out-gassed air are safely avoidable.
In accordance with the present invention, the flow rate is adaptable to the aircraft-side fuel line geometry in terms of both the resultant pressure decrease and the adequate flow velocity, thereby ensuring an appropriate fuel supply throughout the altitude range (sea level to maximum flight altitude).
In accordance with the present invention, a particularly advantageous method to increase the flow rate in the aircraft-side fuel supply lines is to increase the fuel mass flow beyond the normal engine demand. For this purpose, the present invention provides for a circulation system which returns the excess fuel from the engine to the tank. This characteristic is easily achieved since modern engines (aero gas turbines) have a pump capacity which is more than sufficient to cater for this additional mass flow. The large pump capacity results from the fact that the xe2x80x9cwindmilling startxe2x80x9d is often used as the controlling operating point in pump design, i.e. when the engine is started while driven by the air stream. Here, the engine high-pressure pump is required to provide a relatively high pressure at correspondingly low rotational speed, which results in a considerable over-capacity of the pump for all other operating points. The circulation system in accordance with the present invention can, therefore, be easily combined with the usual aero gas turbines.
In accordance with the present invention, the circulation system may be designed such that it taps the fuel circulation flow either downstream of the low-pressure turbine or down-stream of the high-pressure turbine. Both approaches will similarly increase the fuel mass flow in the aircraft-side fuel supply lines, while having different consequences on the thermal situation of the engine. The fuel temperature down-stream of the low-pressure pump is usually only slightly higher than the tank temperature, whereas the fuel temperature on the high-pressure side may well be beyond 100xc2x0 C. due to the heat input from the oil cooler and the high-pressure pump. Which of these approaches is actually applied depends essentially on the overall configuration of the engine/aircraft fuel system. The advantages according to the present design exist in both approaches.
In order to demonstrate the functioning of the method and the system according to the present invention, a scale set-up of the aircraft-side and the engine-side fuel system was made on a test stand, using genuine lines and accessories from a Boeing 717-200 aircraft and genuine BR715 engines. On the test stand, the following climb suction altitude limits were obtained with JET A fuel:
In a subsequent test with a Boeing 717-200, the following climb suction altitude limits were obtained with JET A fuel:
The climb suction altitude limit on the test aircraft, which is lower than that on the test stand, does not constitute a physical limit since higher flight altitudes were not tested. The test stand results, therefore, demonstrate the significant power reserves in the suction altitude limit of the test aircraft in connection with the fuel supply system according to the present invention. In order to demonstrate the stability of the engine operation, the flight tests included dynamic maneuvers, such as banking turns and negative acceleration.
The advantages as provided by the present invention also exist with highly volatile fuel grades, such as JP-4. Here, the suction altitude limit in climb increased from 23.500 ft to 27.000 ft, the suction altitude limit at constant altitude from 25.000 ft to even 37.000 ft.